This invention relates to an apparatus for analyzing the dielectric properties of a sample by the use of parallel plate electrodes.
It is well known that by measuring the dielectric properties of a sample as a function of temperature, valuable information can be gained concerning the physical and chemical properties of the sample. For many years such measurements have been made by placing a sample between parallel plate electrodes, applying an electrical signal to one of the electrodes (i.e. the excitation electrode) and measuring the electrical signal from the other electrode (i.e. the response electrode). The following equation is used: EQU C=e.sub.o e'A/d
where
C=Capacitance PA1 e.sub.o =Permitivity of Free Space (a constant) PA1 e'=Permitivity of Sample (being measured) PA1 A=Area of Parallel Plate Response Electrode PA1 d=Distance Between the Excitation and Response Electrode Plates
By measuring capacitance, the permitivity of the sample (e') can easily be calculated if the area of the parallel plate electrode and the distance between the excitation and response electrodes are known. However, a common dilemma when making these measurements is obtaining an accurate measurement of distance between the plates. This is because most measurements are made as a function of temperature, and the sample changes in dimension as the experiment progresses. However, despite this fact, prior parallel plate dielectric analyzers have usually assumed the distance between the electrodes to be the thickness of the sample at room temperature. Thus, as the material expands or contracts as a function of temperature, the measured values are in error by the factor: ##EQU1## In some instances this error is compensated by allowing for the coefficient of thermal expansion (CTE) of the material (assuming it is known with some accuracy). But this is not an accurate correction since the CTE changes as a material goes through its glass transition. The CTE also assumes zero force on the sample which is not practical when making dielectric measurements on solid samples.
All known instruments either apply a constant force to a sample initially and run the experiment in that mode (constant force), or set a plate spacing and let it remain constant during an experiment (constant distance). In the constant force mode at elevated temperatures, when the sample melts, the two plates come together, short circuit, and the experiment is prematurely terminated. In the constant distance mode, if the sample melts, contact with the top plate is lost, and once again the experiment is prematurely terminated.
Another significant practical problem with conventional parallel plate dielectric analyzers arises because current analyzers use either steel or gold plated metallic plates. After a sample has passed its glass transition T.sub.G (point of interest), it begins to flow, and as it cools it can adhere to the highly-polished, precision-machined plates. Many times plates must be removed from the instrument to scrape samples off. The plates must then be reground to ensure parallelism for the next experiment. This can be a costly and time-consuming operation. One popular alternative is to use a thin release film (i.e. Teflon.RTM., a fluorocarbon polymer) to make sample removal easier. This film, however, influences the measurement of the dielectric properties and limits the experimental temperature to a temperature less than the melting point of the Teflon.RTM. release film. (Ceramic sensors with a gold conductor are used in single plate dielectric analyzers.
Accurate measurements of sample temperatures are also important since dielectric measurements are normally monitored as a function of temperature. In parallel plate dielectric analyzer, typically a thermocouple is placed as close to the edge of the sample and plate as possible without contact, and the sample temperature is assumed to be that of the thermocouple (melting a sample on the thermocouple would require extensive clean up or disposal of the thermocouple after the experiment). Obviously, this temperature measurement is not as accurate as measuring the temperature of the sample directly. (In single plate dielectric analyzers it is known to incorporate a thermal diode in the electrode.)
A dielectric analyzer is needed which can vary the spacing between the electrodes as the sample expands, contracts or melts in order to keep the electrodes in constant contact with the sample. As the electrode spacing is varied, the analyzer must also be able to sense the distance between them so that the dielectric calculations are accurate regardless of electrode spacing. A dielectric analyzer is also needed which has electrodes which are easily replaced if their surfaces become marred. Lastly, a dielectric analyzer is needed which will give accurate temperature measurements of the sample.